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arXiv 2102.13580nucl-th
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形变核低能自旋和轨道磁偶极激发的微观分析

Microscopic analysis of low-energy spin and orbital magnetic dipole excitations in deformed nuclei

V. O. Nesterenko, P. I. Vishnevskiy, J. Kvasil, A. Repko, W. Kleinig

备注：
15 pages, 13 figures
发表信息：
Physical Review C 103, 064313 (2021)
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摘要：

最近，用Wigner函数矩（WFM）方法预测了形变核中的低能磁偶极子$(M1)$自旋剪刀共振（SSR），它位于普通轨道剪刀共振（OSR）之下.我们使用完全自洽的Skyrme准粒子随机相位近似（QRPA）方法来分析这个预测。利用Skyrme力SkM*、SVbas和SG 2研究了$^{160,162,164}$ Dy和$^{232}$ Th的SSR和OSR。该方法的准确性是合理的M1自旋翻转巨共振的一个很好的描述。计算表明，同位素$^{160,162,164}$ Dy在1.5-2.4 MeV（低于OSR）下确实具有$I^πK=1^+1$态，该态具有大的$M1$自旋强度（$K$是总核矩到对称z轴的投影）。这些状态几乎完全耗尽的$pp[411\uparrow, 411\downarrow]$和$nn[521\uparrow, 521\downarrow]$自旋翻转配置对应于$pp[2d_{3/2}, 2d_{5/2}]$和$nn[2f_{5/2}, 2f_{7/2}]$结构的球形极限。因此，预测的SSR实际上是减少到低轨道（l= 2，3）自旋翻转状态。与WFM自旋剪刀图相矛盾的是，形变不是低能自旋$M1$态的主要起源，而只是影响其性质的一个因素。自旋和轨道强度通常是混合的，并表现出干涉：在SSR范围内弱相消，在OSR范围内强相长。在$^{232}$ Th中，发现$M1$自旋强度非常小。在2.4-4 MeV实验上观察到的$^{160,162,164}$ Dy和$^{232}$ Th的两组$I^π=1^+$态主要由轨道强度的碎裂来解释。QRPA态的核流分布部分符合等矢量轨道剪刀流，但不符合自旋剪刀流。

英文摘要：

A low-energy magnetic dipole $(M1)$ spin-scissors resonance (SSR) located just below the ordinary orbital scissors resonance (OSR) was recently predicted in deformed nuclei within the Wigner Function Moments (WFM) approach. We analyze this prediction using fully self-consistent Skyrme Quasiparticle Random Phase Approximation (QRPA) method. Skyrme forces SkM*, SVbas and SG2 are implemented to explore SSR and OSR in $^{160,162,164}$Dy and $^{232}$Th. Accuracy of the method is justified by a good description of M1 spin-flip giant resonance. The calculations show that isotopes $^{160,162,164}$Dy indeed have at 1.5-2.4 MeV (below OSR) $I^πK=1^+1$ states with a large $M1$ spin strength ($K$ is the projection of the total nuclear moment to the symmetry z-axis). These states are almost fully exhausted by $pp[411\uparrow, 411\downarrow]$ and $nn[521\uparrow, 521\downarrow]$ spin-flip configurations corresponding to $pp[2d_{3/2}, 2d_{5/2}]$ and $nn[2f_{5/2}, 2f_{7/2}]$ structures in the spherical limit. So the predicted SSR is actually reduced to low-orbital (l=2,3) spin-flip states. Following our analysis and in contradiction with WFM spin-scissors picture, deformation is not the principle origin of the low-energy spin $M1$ states but only a factor affecting their features. The spin and orbital strengths are generally mixed and exhibit the interference: weak destructive in SSR range and strong constructive in OSR range. In $^{232}$Th, the $M1$ spin strength is found very small. Two groups of $I^π=1^+$ states observed experimentally at 2.4-4 MeV in $^{160,162,164}$Dy and at 2-4 MeV in $^{232}$Th are mainly explained by fragmentation of the orbital strength. Distributions of nuclear currents in QRPA states partly correspond to the isovector orbital-scissors flow but not to spin-scissors one.

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